Determining the unroofing history of basement rocks in a highly-extended terrain using new (U-Th)/He and Ar/Ar thermochronometry: an example from Central Death Valley, CA
The Badwater turtleback (BWT) is one of three highly-exhumed antiformal structures along the western flank of the Black Mountains in Death Valley, California. To address longstanding questions about Cenozoic large-magnitude extension in the area, we collected 10 samples along an E-W transect across the BWT and applied apatite and zircon (U-Th)/He, biotite, muscovite, and K-feldspar Ar/Ar, zircon U-Pb analyses, and K-feldspar multi-domain diffusion modeling to revealing a three-phase cooling history. Slow (<2 °C/m.y.) cooling occurred from ca. 32-6 Ma, followed by rapid (120-140 °C/m.y.) cooling from ca. 6-4.5 Ma, and slower (30-120 °C/m.y.) cooling from 4.5 until the present. When compared to previously published cooling envelopes of the Copper Canyon turtleback and Mormon Point turtleback to the south, a northwest migration of cooling is broadly evident, consistent with a top-to-NW removal of the hanging wall from above a detachment fault. Interpretation of range-wide cooling patterns in the >300 °C ductile crust is complicated by possible thermal perturbations from the voluminous Willow Spring diorite and Smith Mountain granite. Changes in cooling rates at the BWT correlate with a 6 Ma shift from Basin and Range extension to dextral transtension, and a 4.5-3.5 Ma transfer of locus of deformation to the Panamint Valley and eventually Owens Valley fault systems. Modern extension rate measurements support this transfer of deformation. I propose a six-phase tectonic story, beginning with ca. 140-40 Ma Sevier-Laramide orogenesis of a thickened crust. Post-orogenic collapse and erosion dominate from ca. 32-16 causing slow exhumation of the BWT. At 16-14 Ma a detachment fault forms with a breakaway south of the Black Mountains, with listric normal faults in the hanging wall. Basin and Range extension continues from 14-8 generating moderate extension and causing exhumation of the turtlebacks through the brittle-ductile transition. Dextral transtension associated with a change in Pacific plate motions beings at 7-6 Ma, producing a pull-apart basin across the Black Mountains and rapid extension rates within. The locus of deformation transfers to the Panamint and Owens Valley fault systems between 4.5-3.5 Ma, slowing extension in the Black Mountains.